Arcing problems. The how, what, where, and why.

[Joeviocoe]

Can someone from LPP explain what has been happening from an engineering perspective with arcing. What has been attempted to fix it, why it didn’t work… and what the true fix could be (or is currently working).

Thanks.

[delt0r]

Not quite the reply you want(not from LPP). But having done some HV work previously I can comment on the general issues of arcing. Basically any HV things tend to have some issues because insulating is really harder than it looks. Furthermore insulation degrades over time. Expensive HV cables are not rated for very long. There are a lot of physical process that matter at this level. Not all well understood and most highly stochastic. It only takes one electron to get enough energy to knock off more electrons… and now you have a lot of current and heat where you really don’t want it.

Now add the pulse nature and high currents that a DPF has. This further compound problems quite considerably. The biggest issue i would guess is the high stresses that the magnetic fields would place on everything. This means each pulse there are strong forces trying push everything apart, and no matter how strong everything is, things will flex a little. This can then compromise insulation by cause physical damage or by simply providing somewhere to seed a discharge.

A lot of HV equipment has arcing issues. The trick is to not let it destroy the device.

[asymmetric_implosion]

The main problems with high current pulses are contacts at interfaces between pieces and insulation. Insulation can and should last for long periods when properly designed. I operate a plasma focus that has a similar electric field in the gap between the anode and cathode as FoFu-1. It has operated for nearly 1 million shots without any problems. The more common problem is the interface between two pieces. When you think about low voltage, low current connections, simple contact between the metals is adequate. The ohmic loss in the contact is small compared to the rest of the circuit so little voltage is dropped across the contact (10 kA/cm of length for a contact that is only 0.01 inches wide.

The long shutdown is probably due to switch problems. The contact issue typically takes a couple weeks to resolve at most. The new switches are probably slow to fab (16 weeks is not unusual). Once at LPP, they must be installed and tested. It takes time to get the switch jitter down to tolerable levels.

[Lerner]

We’ll have a report soon. We are still dealing with arcing–jsut a question of getting all the best practices right. We are trying out indium wire, but it takes a bit of experience.

[asymmetric_implosion]

Good luck with Indium wire. We found it to be a pain. Also, it activates pretty nicely with neutrons.

Rather than wire I suggest thin walled copper tube. It yields easily, it’s cheap and it has none of the problems of Indium.

[Lerner]

What problmes did you have with indium? Our first try we did omit cleaning with HC acid to get irid of the InO layer, but will do that this time. We don’t se how we can get enough pressure for copper. We are alreayd using a copper contact and still get arcing.

[asymmetric_implosion]

Our experience with indium contacts was they tended to melt and solder the parts together.

If you can share, what is the size of the contact (thickness and diameter if circular)? You might need to move to a larger diameter contact or a contact with more surface area and many more bolts. I know we doubled the bolts in our tightening pattern at one point and it dramatically improved our contact reliability. I think the preferred strategy is to use many smaller bolts rather than a few larger ones. This leads to a more uniform contact like a conflat flange. Are you tightening the bolts in a pattern that leads to smooth and uniform draw-dwon on the contact? If you are arcing at one location, it can be due to a bad tightening pattern.

The other question that might be relevant is the inductance of the contact area. Are you sure that inductance isn’t the problem at the contact and the current is jumping the contact in favor of air or fuel gas leading to discharge.

[Lerner]

We did already double the number of bolts to 32 and used titanium bolts for strength. Our contacts are about 3” in diameter and 70 mil in width. The basic problem it seems to me is the oxide on the surfaces. Coolamp can take case of the copper oxide. According to the literature, indium oxide can be removed with weak hydrochloric acid but I don’t see what can remove the chromium oxide from the surface of stainless steel. Any suggestions there?

[asymmetric_implosion]

We seldom have problems with stainless steel contacts. We typically connect SS304 plates to silver plated aluminum and copper with little trouble. If you want the SS304 surface to behave you could have it nickel plated. We considered nickel plating at contacts between SS304. Another approach is brazing copper onto the SS304 which makes a good contact at the SS304 copper interface and make the current contact between two interfaces of silver plated copper.

Using stronger bolts does not guarantee better pressure. The strength is determined by the strength of the threads into what I hope is the steel.

If the arc is always in the same location you might have a low spot on one of the surfaces. A nice face cut could help. You need to make sure the steel bites into the copper by more than the machine tolerance for the flatness. We always have a perceptible (by touch) indention even after the first contact. We’ve changed cathodes, anodes and other pieces over and over without any problems.

[Lerner]

We’ll look at nickel plating. what pressure do you apply to the contacts ? I calculate we use 135 Mpa

[Lerner]

We decided to go with gold over nickel. That will ensure no oxides. It seem to me that the chromium oxide on stainless pretty much guarantees there can’t be full contact. Now that I know about that, I am surprised you have done OK with unplated stainless. Maybe it is because you are running under 0.4 MA/cm^2 and we are running a bit above that–0.5 MA/cm^2. The gold should ensure we can go up to 1 MA/cm^2 at full power.

[asymmetric_implosion]

Gold is a great choice to minimize oxide. We didn’t want to deal with plating gold and it turned out we didn’t need to.

Our anode plate is SS304. Our anode is SS304. We use a small copper gasket (no silver plating) between the two surfaces. The anode contact diameter is 0.75″ and 30 mil thick. At 300 kA which is our normal upper limit of operation in rep rate mode, that translates to 0.63 MA/cm^2. We’ve fired a 450 kA shots at low rep rate without any problems or ~1 MA/cm^2. I’ve run over 20,000 shots on that contact without issue at 300 kA. We use a single 1/4″ screw to hold the anode. Our cathode contacts are far larger, more like 4″ with 30 mil thicknesses using SS304 on silver plated Al. We’ve also done Cu on SS304, W on SS304 and Moly on SS304 up to 400 kA/cm^2 when the harder metal has the contact.

I haven’t done the pressure calculations since the contacts are working well.

[mchargue]

Thanks for carrying on this discussion here, folks. It’s fascinating, and feels like a chapter out of a John W Campbell book. I also feel like everyone here should chip in for a plane ticket for asymmetric so that you guys can sit down & talk this out. Sign me up to contribute to that!

In the meantime, I’ll enjoy the posts between the two of you, and reading what you both write individually.

Patrick

[asymmetric_implosion]

I hope it useful but enjoyable is a nice sidelight. Our first machine and FoFu-1 were designed by the same person, well, between the switches and the plasma load. Our first machine has been operating since 2006 so we have a few more years “practical” experience. We spent time burning contacts and revising. I know the pain all too well. My experience is limited to 0.5 MA so effects unique to larger machines are beyond my experience at the moment. I hope that will change next year.

Our interest is rep-rate PF operation so we had to focus on making the source run at 1 Hz. Contacts are a key part of the problem. We run our small PF at 10 Hz for hours so there is another level of problems that had to be addressed. This develop was evolutionary instead of revolutionary so it took us quite a few years to get everything running just OK. We hope to continue our PF work to make the source exceptional for our customers.

[Joeviocoe]

This is why the Internet is great. Collaboration to find solutions. I am so glad that this forum can help introduce new ideas that may help Focus Fusion to overcome this arcing snag. I want to hear about burning Boron-11 already! And I still hope that within the next 6 months, announcements can be made regarding the final disposition of feasibility of the Focus Fusion approach.

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